1. Field of the Invention
The present invention relates to an optical pickup and a recording and/or reproducing apparatus for optically recording and/or reproducing information on an information medium.
2. Description of the Related Art
An optical disc such as a CD (Compact Disc) or DVD (Digital Video Disc or Digital Versatile Disc) is known as an information medium used for optical information recording and reproduction.
The optical disc has a multi-layered structure including a recording layer, a light transmitting layer, and a protection layer. The recordation and reproduction of information data is done by irradiating a light beam onto the recording layer through the light-transmitting layer.
FIG. 1 illustrates a configuration of a known optical pickup for such recordation and/or reproduction of information as mentioned above. Referring to the figure, the optical pickup includes a light source 100 for emitting a linear polarization laser beam, a collimator lens 101, a beam splitter 102, a quarter-wave plate 103, an objective lens 104, a condenser lens 105, and an optical detector 106.
A linear polarization laser beam (i.e., laser beam for information recording) which is modulated by information data to be recorded is emitted from the light source 100 to record the information data on the optical disc. The laser beam is, then, collimated by the collimator lens 101 to pass through the beam splitter 102. Subsequently, the laser beam is circularly polarized by the quarter-wave plate 103 and then converged by the objective lens 104. The laser beam is thereby provided with a small diameter for radiation (hereinafter referred to as the main beam) to radiate the recording layer of the optical disc 107 through the light transmitting layer. This allows the information data to be recorded on the recording layer by the optical energy of the main beam.
In addition, a laser beam for reading information is emitted from the light source 100 to read information on the optical disc. The laser beam for reading information has a small diameter of radiation to form the main beam and is allowed to radiate the recording layer of the optical disc 107 through the light transmitting layer as in the case of recording information. Moreover, reflected light produced by radiating the optical disc 107 with the aforementioned main beam passes through the objective lens 104 and is then polarized by the quarter-wave plate 103 to be orthogonal to the laser beam emitted from the light source 100. The reflected light is thereby reflected by the beam splitter 102 toward the condenser lens 105 and then converged by the condenser lens 105 to be received by the optical detector 106, thereby allowing the information to be read.
An information recording medium having a higher recording density and a larger capacity has been in increasing demands. To respond to the demands, an attempt has been made to increase the numerical aperture NA of the objective lens.
However, an increase in the numerical aperture NA of the objective lens would not allow the aforementioned known pickup to detect the reflected light with high accuracy due to birefringence resulting from the optical characteristics of the light transmitting layer and the recording layer of the optical disc. Thus, different difficulties to be solved have occurred, such as in implementing appropriate reading of information and implementing appropriate servo control for recording information.
More specifically, the optical pickup shown in FIG. 1 allows a linear polarization laser beam to be emitted from the light source 100 and pass through the beam splitter 102. Then, the laser beam is circularly polarized by means of the quarter-wave plate 103 to form a circularly polarized laser beam with which the optical disc 107 is radiated. Subsequently, circularly polarized reflected light produced by the radiation of the laser beam is polarized by the quarter-wave plate 103 to form a linear polarization laser beam. The resulting laser beam is reflected by the beam splitter 102 and then detected by the optical detector 106 via the condenser lens 105.
The presence of birefringence in the optical disc 107, however, causes the reflected light to be elliptically polarized whereas a circularly polarized light beam is to radiate the optical disc 107. Accordingly, every light beam is not linearly polarized when the elliptically polarized reflected light passes through the quarter-wave plate 103. Part of the reflected light passes through the beam splitter 102 to return to the collimator lens 101, whereas the remaining linear polarization reflected light is reflected by the beam splitter 102 to reach the optical detector 106. This makes it impossible to detect the due amount of reflected light on the optical detector 106, thus resulting, for example, in a decrease in S/N. In addition, it is necessary to take countermeasures against an adverse effect to be exerted, by the part of the reflected light through the collimator lens 101, on the semiconductor laser diode provided in the light source 100.
In particular, an increase in numerical aperture NA of the objective lens 104 to provide the optical disc 107 with a higher density would cause the range of the angle of the light beam incident to the optical disc 107 to become larger. This causes the distribution of birefringence, which is dependent on the incident angle, to be more spread in the pupil plane of the disc, thus making the effect of the birefringence greater to such an extent that it cannot be neglected. In addition, with the aforementioned prior-art optical pickup, use of a short-wavelength laser beam to implement a higher density would make it difficult to reduce the effect of the birefringence in the optical disc 107.
The present invention was made in view of the above problems. It is therefore an object of the present invention to provide an information recording and/or reproducing apparatus and an optical pickup, which can prevent the effect of birefringence to allow highly accurate recording and reading of information.
It is another object of the present invention to provide an information recording and/or reproducing apparatus and an optical pickup, which can prevent the effect of birefringence to allow highly accurate recording and reading of information even when an objective lens with a greater numerical aperture or a laser beam of a shorter wavelength for reading or recording information is employed.
According to the present invention, there is provided an optical pickup including a light source, an optical element having a polarization characteristic of allowing transmittance or reflectance to be changed according to a polarization of incident light and an objective lens, wherein a light beam emitted from the light source is transmitted or reflected in one direction of the optical element and converged by the objective lens to radiate an information medium therewith, and the reflected light beam from the information medium passes through the objective lens to be incident on the optical element in another direction thereof as well as being reflected by the optical element to perform optical detection, which comprises a variable optical unit, disposed between the optical element and the objective lens, for providing incident polarized light with a phase difference.
According to another aspect of the present invention, the optical pickup further comprises control means for variably controlling a phase-shift amount of the variable optical unit, wherein the control means controls the phase-shift amount of the variable optical unit according to birefringence of the information medium.
According to another aspect of the present invention, the control means controls the phase-shift amount of the variable optical unit according to birefringence of each information medium.
According to another aspect of the present invention, the variable optical unit is formed of a photoelectric element having a photoelectric effect to be changed in accordance with an applied voltage.
According to another aspect of the present invention, the variable optical unit is formed of a liquid crystal element having a photoelectric effect to be changed in accordance with a voltage applied thereto.
According to the present invention, there is provided an information recording and/or reproducing apparatus having the optical pickup, wherein the light source emits light for reading information and the reflected light from an information medium is optically detected when reading information, and the light source emits light for recording the information when recording information.
According to the above-described optical pickups, the amount of phase-shift of the variable optical unit is adjusted according to the birefringence of the information medium. It is thereby made possible to polarize the reflected light from the information medium in the direction in which the optical element reflects the reflected light, thus optically detecting the entire amount of the reflected light.
A light beam emitted from the light source passes through the optical element to be incident on the variable optical unit. The light beam is polarized according to the phase sift amount which corresponds to the birefringence of the information medium when passing through the variable optical unit and converged by the objective lens to radiate the information medium.
Subsequently, the light beam which has radiated the information medium is reflected by the information medium. The reflected light passes again through the objective lens and is then transmitted from the variable optical unit to be incident on the optical element. The reflected light is further polarized according to the phase-shift amount which corresponds to the birefringence of the information medium when the aforementioned reflected light passes through the variable optical unit.
The reflected light, which has passed through the variable optical unit, is thereby polarized in such a direction in which the optical element can reflect the reflected light, so that the reflected light is further reflected by the optical element to be detected. Consequently, it is made possible to detect the substantially entire amount of the reflected light, thus enabling improvement in accuracy of reading information. In addition, it is made possible to detect the substantially entire amount of the reflected light when the reflected light is optically detected to generate various servo signals in order to record information, thereby improving the accuracy when recording information.
In particular, the present invention provides an optical pickup which is suitable for preventing the effect of birefringence in the information medium where an objective lens with a greater numerical aperture or light of a shorter wavelength emitted from the light source is employed to provide the information medium with a higher density.
According to the above-described configuration, the aforementioned optical pickup makes it possible to prevent the effect of birefringence in the information medium. Thus, such an information recording and/or reproducing apparatus can be provided which is adapted for an information medium with a high density.